Thixotropic gas producing gel

ABSTRACT

A combustible gas producing gel having a viscosity up to about 200 poise at a 100 reciprocal second rate of shear and a temperature of minus 65* F. and being thermally stable at temperatures up to about 165* F. comprising the combination of solid consumable particles suspended in a hydrocarbon carrier having eight to 10 carbon atoms and containing an arganoaluminum phosphonate gelling agent. The solid consumable particles may be present in a concentration of from about 77 percent to about 95 percent by weight based upon the total weight of gel; the hydrocarbon carrier may be present in a concentration of from about 5 percent to about 23 percent by weight based upon the total weight of the gel; and the weight ratio of the hydrocarbon carrier to the gelling agent is less than about 100. The solid consumable particles may be all oxidizer particles or a combination of oxidizer and fuel particles.

[ 51 Mar. 28, 1972 [54] THKXQTROPHC GAS PRODUCING GEL [72] Inventor:Herbert A. Bar-tick, Alexandria, Va.

[73] Assignee: The Susquehanna Corporation, Fairfax County, Va.

[22] Filed: Aug. 25, 1969 [21] Appl.No.: 852,904

[52] ILLS. Cl ..149/42, 60/217, 149/18, 149/21, 149/22, 149/36, 149/44,149/75, 149/75,

[51] lint. Cl ..C06b 11/00, C06b 19/04 [58] Field 0llSeareh.......149/18, 19, 21, 22, 42, 44, 149/76,112,113,114,115, 75, 87, 89; 60/2173,507,719 4/1970 Hodgson ..149/18 X Primary Examiner-Leland A. SebastianAttorney-Martha Ross [57] ABSTRACT A combustible gas producing gelhaving a viscosity up to about 200 poise at a 100 reciprocal second rateof shear and a temperature of minus 65 F. and being thermally stable attemperatures up to about 165 F. comprising the combination of solidconsumable particles suspended in a hydrocarbon carrier having eight to10 carbon atoms and containing an arganoalu minum phosphonate gellingagent. The solid consumable particles may be present in a concentrationof from about 77 percent to about 95 percent by weight based upon thetotal weight of gel; the hydrocarbon carrier may be present in aconcentration of from about 5 percent to about 23 percent by weightbased upon the total weight of the gel; and the weight ratio of thehydrocarbon carrier to the gelling agent is less than about 100. Thesolid consumable particles may be all oxidizer particles or acombination of oxidizer and fuel particles.

7 Claims, No Drawings TIHIHXOTE'XOMC GAS PRODUCING GEL BACKGROUND OF THEINVENTION Compositions capable of generating gases containing largeamounts of available energy for such purposes as producing thrust, heator gas pressure may be divided generally into monopropellants which arecompositions that are substantially self-sufficient with regard tooxidant requirements and bipropeliants wherein a fuel is maintainedseparately from an oxidizer source until admixture at point ofcombustion.

in oxygen deficient atmospheres, monopropellants possess considerableadvantage over bipropellants and both liquid and solid monopropellantsare used extensively. For use, liquid monopropellants require only oneset of equipment such as a storage tank, propellant pump, feed lines andvalves thereby eliminating elaborate systems which are necessary toensure proportioned flows of the separate components of fuel andoxidizer in bipropellant systems and. their adequate mixing incombustion chambers. Liquid monopropellants may be injected into acombustion chamber in the form of finely divided droplets or sprays togive mass burning rates which may be controlled by varying the rate ofinjection. Combustion can be stopped by shutting off the flow andresumed at will, and performance is not dependent generally upon thetemperature environment of the system. Further, operation is limitedonly by the capacity of the storage tanks or reservoirs, and combustionchambers need to be large enough only to provide sufficient space forcompletion of the combustion reaction.

The known liquid monopropellants are characterized by certaindisadvantages such as low density, low specific impulse, high toxicity,and excessive sensitivity to heat and shock, all which may cause lowperformance, detonation and corrosion of various parts of the propellantsystem such as valves and lines. When liquid monopropellants are used asfuels for rocket motors, unburned droplets of the liquid propellant mayleave the combustion chamber and be cooled during expansion in thenozzle before combustion occurs lowering performance of the rocket motorsystem. Also, the attitude of the system may affect the performance ofthe system. in certain liquid monopropellant systems, catalyst beds arerequired for combustion, and vibration of the rocket motor often makesretention and fixing of the catalyst bed in the combustion chamberdifficult. Storage and transportation of liquid monopropellants also aredifficult because of ever present leak hazards of both fire andtoxicity.

Solid monopropellants possess the advantages of high density, low heatand shock sensitivity, good stability, long storage ability, absence ofleakage problems, low corrosiveness and toxicity, and propellant fillingand injection equipment is not needed since all of the solid propellantgenerally is contained directly in the combustion chamber. Solidmonopropellants do not require purgings of systems after firings; do notneed any external combustion catalyst; and are not affected by theattitude of the system. Solid monopropellants, however, do possess anumber of their own disadvantages.

The solid grain must be sufficiently strong and free from mechanicalflaws so that cracking or shattering does not take place under pressureor vibrational stresses; and many solid propellants tend also to becomeexcessively brittle at low ambient temperatures which reduces fractureresistance. If solid propeilants are fractured, burning surfaces may beincreased causing uncontrolled burning in the combustion chamberresulting in pressures exceeding the design pressures of the combustionchamber walls and complete failure of the propellant system.

Although burning solid monopropellants may be quenched, if necessary,reignition generally is not feasible and unburned portions may be atotal loss. Intermittant operation of a motor system containing solidmonopropellants, therefore, is generally impractical, and the ambienttemperature of a solid propellant is an important parameter indetermining burning rate which cannot be compensated for during use byvariation of the area of the burning surface in that solidmonopropellants must be predesigned with respect to burning surface areafor each particular application.

Further, when using solid monopropellants, a combustion chamber must beof sufficient size to accommodate all of the propellant and is generallylarger than that required for combustion of a liquid propellant. Thisrequires the walls of the entire combustion chamber to be sufficientlystrong to withstand high combustion gas pressures and to be completelyinsulated, or otherwise cooled, to withstand the high combustion gastemperatures, both of which may cause a serious weight problem innon-stationary applications.

It is obvious, therefore, that a monopropellant which has the advantagesof both a liquid monopropellant and a solid monopropellant, while havingsubstantially none of the disadvantages of either, is a worthwhileadvance in the art. Motor systems could be started and stopped asdesired and the attitude of the system would not affect its operation.Catalyst beds could be eliminated. Leakage problems would benonexistent. Temperature sensitivity could be eliminated, and smallcombustion chambers could be used. Further, low heat and shocksensitivity with attending good stability for long periods would permitease in handling and storage. Therefore, a thixotropic monopropellantgas producing gel having a low viscosity at low temperatures and beingthermally stable at high temperatures is a worthwhile advance in theart.

SUMMARY OF THE INVENTION In accordance with this invention, there isprovided a thix otropic combustible gas producing monopropellant gelhaving a low viscosity up to about 200 poise at a reciprocal second rateof shear and a temperature of minus 65 F. and which is thermally stableat temperatures up to about F. The monopropellant gel comprises solidconsumable particles suspended in a hydrocarbon carrier having eight to10 carbon atoms and containing gelling agent. The solid consumableparticles are present in an organoaluminum phosphonate a concentrationof from about 77 percent to about 95 percent by weight based upon thetotal weight of the gel; the hydrocarbon carrier is present in aconcentration of from about 5% to about 23% by weight based upon thetotal weight of the gel; and the weight ratio of the hydrocarbon carrierto the gelling agent is less than about 100. The solid consumableparticles may be oxidizer particles or a mixture of oxidizer and fuelparticles and may be present in a particle size range of from about 5microns to about 300 microns in one or more modal distributrons.

The advantages of the thixotropic combustible gas producingmonopropellant gel of this invention are myriad. The monopropellant gelmay be injected or sprayed into small combustion chambers in the mannerof liquid monopropellants and large high-strength combustion chambersand elaborate systems of duplicate pumps and lines are not necessary.Also, storage, handling, thermal stability and shock resistance problemscommon to usual liquid monopropellants are not present in themonopropellant gel of this invention. Moreover, these advantages are allpresent in the monopropellant gel of this invention over a widetemperature range of from about minus 65 F. to about 165 F. Otheradvantages will be readily apparent to those skilled in the art from themore detailed description of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS The monopropellant gel of thisinvention comprises solid consumable particles suspended in ahydrocarbon carrier having eight to 10 carbon atoms and containing anorganoaluminum phosphonate gelling agent. Solid consumable particles maybe present in a concentration of from about 77% to about 95% by weightbased upon the total weight of the gel and may comprise oxidizerparticles or a mixture of oxidizer and fuel particles. The oxidizerparticles may be any of well-known oxidizers such as ammoniumperchlorate, hydroxyl ammonium perchlorate, hexanitromethane, hydrazineperchlorate,

hydrazine diperchlorate, hydrazine nitroform and mixtures thereof. Whenonly oxidizer particles are present in the gel, it is preferred thatammonium perchlorate particles be used.

When fuel particles are used in combination with oxidizer particles inaccordance with this invention, any of the wellicnown particulate fuelsmay be used such as beryllium, beryllium hydride, aluminum, aluminumhydride, boron, boron hydride, zirconium and mixtures thereof. When fuelparticles are used in combination with oxidizer particles in accordancewith this invention, it is preferred that aluminum or boron particles,or a mixture thereof, be used in the gel.

The solid consumable particles used in accordance with this inventionmay be of substantially any shape; however, it is preferred that thesolid consumable particles have an average particle size of from about 5microns to about 300 microns. It is preferred also that the particles bespherical or substantially spherical. The solid particles may be presentin the gel in one or more modal distributions and when the solidparticles are present in a bimodal distribution, it is preferred thatthe larger particles comprise from about 60% to about 80% of the totalweight of particles.

The hydrocarbon carrier of this invention is present in the gel in aconcentration of from about 5% to about 23% by weight based upon thetotal weight of the gel. The hydrocarbon liquid must be a saturatedhydrocarbon to prevent attack by the oxidizer component. A preferredsaturated hydrocarbon liquid is one containing eight to 10 carbon atomssuch as trimethylhexane and nonane. Of these 2,2,5-trimethylhexane ispreferred.

The organoaluminun phosphonate gelling agent used in accordance withthis invention is compatible with the hydrocarbon carrier and notsusceptable to degradation or attack by the oxidizer component. Thegelling agent should be present in the gel in a concentration sufficientto provide a weight ratio of hydrocarbon carrier to gelling agent ofless than about 100. When the hydrocarbon carrier used in accordancewith; this invention is trimethylhexane, it is preferred that theigelling agent be aluminum tri(monobutyl-Z-thiadodecyl) phosphonate. incertain gel formulations, the addition of a wetting agent to thehydrocarbon carrier may be necessary or desired for good gel physicalproperties to permit better wetting of the solid particles with thehydrocarbon carrier. When a wetting agent is used, it is preferred thatsufficient, wetting agent is provided to permit a substantiallyimonomolecular layer of wetting agent to be formed on the surface of thesolid combustible particles. Also, it is necessary that the wettingagent be extremely resistant to attack or degradation by the oxidizercomponent and compatible with other gel components. Wetting agents whichare suitable for use in accordance with this invention are materialssuch as sorbitan sesquioleate, hydrogenated coconut oils, syntheticcoconut oils and like materials. When ammonium perchlorate is used asoxidizer, and trimethylhexane is used as hydrocarbon carrier, it ispreferred that sorbitan sesquioleate be used as the wetting agent.

EXAMPLE 1 as the hydrocarbon carrier and the weight ratio of ammoniumf75' cles perchlorate to trimethylhexane was varied from 75/25 to 95/5The resuits of this determination are shown in Table I.

TABLE I [Specific impulse (vacuum) at 100 p.s.i. combustion chamberpressure and an external pressure of 0.1-5.0 p.s.i.]

Weight ratio, AP/TMH 76/25 80/20 /15 88/12 90/ 10 /5 Expansion Ratio:

As can be seen from the above Table 1, maximum values for performancewere obtained at weight ratios of AP/l M15 in excess of 75/2 5 andiessthan 95/5 at expansion ratios. W

or r- EXdM LFEH Performance values were obtained in the manner ofExample I with all variables being the same except that a combustionchamber pressure of 200 p.s.i. and an external pressure of 0.1-10 p.s.i.were used. Results of this determination are shown in Table 11 below.

TABLE II [Specific impulse (vacuum) at 200 p.s.i. combustion chamberpressure values were obtained at weight ratios in excess of 75/25 andless than 95/5 for'all expansion ratios.

.. a v .7 flAMBIaEJlL- V H Performance determinations in the manner ofExamples 1 and 11 were made at weight ratios of solid consumable partilcles to hydrocarbon carrier of 85/15 and 88/12 wherein a portion of thesolid consumable particles were replaced with fuel 1 particles. Thecombustion pressure was p.s.i., the external pressure was 0.1-5.0 p.s.i.and specific impulse (vacuum) was determined for expansion ratios of tento 80 for weight ratios i of 75 AP, 10 aluminum (AL,) 15 trimethylhexane(85% solid consumable particles to 15% hydrocarbon carrier) and 70 AP,15 Al and 15 TMH (85% solid consumable particles to 15% hydrocarboncarrier) and 66 AP, 22 Al, 12 TMl-l (88% solid consumable particles to12% hydrocarbon carrier). The

TABLE 111 Specific Impulse (vacuum) at 100 p.s.i. combustion i chamberpressure and an external pressure of 5 0.1-5.0 p.s.i.

E Expansion Weight Ratio-AP/Al/TMH t Ratio 75/l0/l5 70/15/115 66/22/11210 277 200 zen i 20 288 2% 304 l 40 298 307 318 i so 304 312 2124 so 306316 329 i As can be seen from the above Table 111, good performance 5values were obtained at all expansion ratios for both the 85% aui srg muuashraeelr aise,

EXAMPLE 1V n,

of 20 microns, 11.25% aluminum having an average particle size of 30microns, and 3.75% aluminum having an average particle size of 6microns. These consumable particles were mixed with a hydrocarboncarrier, 2,2,5-trimethylhexane (TMlrl) at a concentration of 14.8% byweight for Composition A and 14.61% by weight for Composition B. Awetting agent, sorbitan sesquioleate, was added in the sameconcentration to both Composition A and Composition B. A gelling agent,aluminum tri(monobutyl-2-thiadodecyl) phosphonate, was added to bothComposition A and Composition B in a concentration sufficient to providea weight ratio of hydrocarbon carrier to gelling agent of 99 inComposition A and 43 in Composition 8. Table IV below shows thecomponent concentration for Compositions A and B.

TABLE IV Thixotropic Gas Producing Gels Gel Composition Wt. 36

Oxidizer (AP) 70.00 70.00 Fuel (Al) 15.00 15.00 Hydrocarbon Carrier(TMH) 14.80 14.61 Wetting Agent 0.05 0.05 i-lC/Gelling agent wt. ratio99 43 Both Composition A and Composition B were thermally soaked in anidentical manner at 165 F. for 10 days to determine their hightemperature stability. Results of this testing showed that Composition Ahad separated over 2.1% of the totai weight whereas Composition B had atotal separation of 0.18% of its total weight demonstrating theunacceptable thermal stability of Composition A and the good thermalstability of Composition B.

EXAMPLE V A thixotropic gas producing gel identical with Composition 18above was prepared having the formulation shown in Table V below.

TABLE V Thixotropic Gas Producing Gel Wt. 2 AP (200 microns) 52.50 AP(20 microns) 17.50 (/11) (30 microns) 11.25 (Al) [6 microns) 3.75 TMH14.61 Wetting Agent 0.05 Gelling Agent 0.34

EXAMPLE V1 A portion of the gas producing gel of Example V above wasthermally soaked for 10 days at 165 F. and compared with a sample of thesame material stored at room temperature for 10 days. The results of thecomparison showed that there was a 17% decrease in viscosity at highshear rates and no significant change at lower shear rates due tothermal soaking for 10 days at 165 F.

EXAMPLE Vll The gas producing gel of Example V above was subjected to asea level motor test in a laboratory test engine and it was found thathigh combustion efficiencies were obtained with smooth ignition when thegel was sprayed into the combustion chambers. A thrust value of 472pound/feet at sea level, a gel flow rate of 0.86 pounds/second, acombustion chamber pressure of 653 psi. absolute were measured duringthe motor test.

EXAMPLE V111 A sample of the gel of Example V above was subjected toimpact testing under a liquid test method conforming to ICRPG LiquidPropellant Test Methods Test No. 4 Results of this test showed that thegel had an impact (liquid test) E equal to 21 kg. cm. which demonstratedthe gel was relatively insensitive to this type of impact initiationeven in the heavy confinement of a liquid tester when compared to impactsensitivity of n-propyl nitrate of 8.4 kg.cm. when performed by the sametest.

EXAMPLE 1X A sample of the gel of Example V was subjected to fiictiontesting to determine the sensitivity to initiation by friction on asliding friction machine. The machine contained a pneumaticallypressurized arm which applied a set force to a sliding plate containinga sample of the gel. A pendulum dropped from various angles impacted theplate containing the sample causing it to slide horizontally beneath anapplication arm for a distance of one inch. The comparative frictionsensitivity is determined by the zero initiation level or the lowest armpressure which will result in initiation of the sample. The initiationcan be an ignition, spark, explosion or decomposition. In testing, thependulum is swung from 45 to impact the plate to reduce the arm pressurefrom 2,000 psi. down to the highest value to which ten consecutive testsfail to ignite the sample. The results of this test showed that the gelof Example V above had a friction test pressure of 600 lb. ft. which isrelatively insensitive to initiation by friction when compared to safetymatch compositions which have a test pressure of 20 lb. ft.

EXAMPLE X Shock testing, in accordance with Specification T01 lA-l -47Adated Jan. 22, 1968, was used to determine the shock sensitivity of asample of the thixotropic gel of Example V above in accordance with thestandard card gap test for solids. Under these rigorous test conditions,negative tests were obtained at zero gap indicating the gel of Example Vwas insensitive to shock.

EXAMPLE XI A standard autoignition test was performed on a sample of thethixotropic gel of Example V and the result showed that a temperaturegreater than 315 C. was needed to autoignite the el. g lclaim:

1. In a combustible gas-producing, monopropellant gel comprising liquidhydrocarbon, solid oxidizer or mixture of solid oxidizer and solid fuel,and gelling agent, the improvement wherein:

a. the monopropellant gel has a viscosity up to about 200 poise at areciprocal rate of shear and a temperature of minus 65 F. and isthermally stable at a temperature up to about F.,

b. the liquid hydrocarbon is a saturated hydrocarbon containing fromeight to 10 carbon atoms; and

c. the gelling agent is an organoaluminum phosphonate.

5. The monopropellant gel of claim 2 wherein the liquid hydrocarbon istrimethyl hexane.

6. The monopropellant gel of claim 3 wherein the liquid hydrocarbon istrimethyl hexane.

7. The monopropellant gel of claim 3 wherein the oxidizer is ammoniumperchlorate and the solid fuel is aluminum.

2. The monopropellant gel of claim 1 wherein the gelling agent isaluminum tri(monobutyl-2-thiadodecyl) phosphonate.
 3. The monopropellantgel of claim 1 which contains in addition sorbitan sesquioleate wettingagent.
 4. The monopropellant gel of claim 1 wherein the liquidhydrocarbon is trimethyl hexane.
 5. The monopropellant gel of claim 2wherein the liquid hydrocarbon is trimethyl hexane.
 6. Themonopropellant gel of claim 3 wherein the liquid hydrocarbon istrimethyl hexane.
 7. The monopropellant gel of claim 3 wherein theoxidizer is ammonium perchlorate and the solid fuel is aluminum.